Compressed gases are used in many industries, including heath care, chemicals, and manufacturing. To provide a compressed gas in a convenient format, compressors are used to reduce the volume of the gas. Compressors useful for compressing a gas include multiple stage compressors, as well as more than one compressor placed in series. Thus, the pressure of a process fluid can be gradually increased from an inlet pressure to a final outlet pressure.
Cryogenic liquids provide a benefit to distribution because of their high density at low pressure. These liquids are transported and stored as liquids but are often used as gas at ambient temperature. Cryogenic pumps or compressors are commonly used, and it is for this purpose the invention is intended.
Compressors for compressing cryogenic gas are well known in the art and are often used to compress boil off gas from cryogenic storage tanks and in air separation processes. Such compressors can consist of a single compression stage or can have multiple compression stages. For multiple stage compressors, the gas being compressed (“process stream”) can be cooled between stages or can be passed on to the subsequent stages without cooling. In systems having interstage cooling, it is known to cool the process stream of a multi-stage compressor using a cryogenic heat sink. It is also known to use heat exchangers between compression stages in which the process stream is cooled against a cryogenic fluid, such as liquid natural gas (LNG). All of these systems, however, are designed to be cooled down to a steady state temperature prior to normal operation and are not designed to operate normally during this transient period.
During the transient period, the process fluid of a cryogenic compressor or pump is not compressed and is generally vented off, resulting in a loss of process fluid. Such venting is terminated when the compressor stages and associated fluid systems have cooled to a temperature that allows the machine to operate in cryogenic mode. Such venting of the process stream is undesirable because of the loss of product and associated cost.
Accordingly, there is a need for an improved system that is capable of operating in a transient cool-down period without venting the process stream.